One at a time was TWO much trouble last time. As in I had to do one of them twice because the alignment wasn't. What is the simplest way to get the two lined up and perpendicular to the frame prior to brazing? A C-clamp is not working for me.

I've never used a fixture for lever bosses on frames I build at home. I scribe a centerline on both sides of the tube, measure the distance from the headtube to where the boss should go, flux everything up and place the boss centered on the lines, and braze. A little weight on the boss (like the handle of an old file) will keep the boss from wandering as the flux boils. More weight risks distorting the tube.

We used a fixture for lever bosses at Trek, but that was so we could crank out dozens of them. For a one-off frame, I wouldn't bother.

Do one at a time. Locate and braze one side, clean up enough to prep the other side and do that one next.

Long ago I made a simple fixture for 28.6 tubes to hole both lever bosses at the same time. Driven by wanting to make it easier/faster. Didn't use it for many years and finally recycled it into other tools. Just a couple of years ago I could have used it again... Andy

This is how I'm doing it on a current project: If you follow the link you'll see the development of it. I am scratching a line down the side of the DT, then aligning a center mark at both ends to make sure it's straight and even to the other side.
by https://www.flickr.com/photos/54319503@N05/, on Flickr

I just use the Mark I eyeball. After tacking pick the frame up and have a good look round from every angle to make sure. Maybe it would help to just hold them in position first with one hand and do a couple of little sharpie marks because it might be easier to see in that position.